“Your Friend” could have an allergy to latex. She might try finding underwear in which the elastic is enclosed in cotton. Also she might try a larger size. Victoria’s Secret cotton undies are very friendly.
Another cause could be chiggers. These microscopic insects crawl up until they can’t crawl anymore, and then they bite and die. Hence they often leave a rash around underwear elastic. The itch goes away in about three weeks, but until then, it is miserable! They usually strike during warm weather, however, so unless she lives in the southern hemisphere (do chiggers live in the SH?), or unless she has been hiking around some warmer meadows, it’s unlikely.